Dense ceramic tubes are used in solid oxide fuel cells and as gas separation membranes, in particular for oxygen separation from air. Other uses are in water electrolysis and sensors. The ceramic material can be a solid electrolyte (SE), which conducts ions and has a negligible electron/hole conductivity. An example is YSZ (Y2O3|x stabilized ZrO2|1-x, x˜0.1), which conducts oxygen ions. SE's also serve in electrolyzers and in sensors that measure characteristic cell voltage. They also serve as gas separation membranes with ions of the filtered gas driven through them by an applied voltage. Dense tubes made of mixed ionic electronic ceramic (MIEC) conductors serve in gas separation membranes. The gas is driven through them by creating a pressure difference of the filtered substance, e.g. oxygen. In all these cases the selectivity is achieved by the nature of the ceramic material used. Thus, for the filtration of oxygen, certain oxides are used that allow only oxygen to diffuse through them in the form of ions accompanied by electrons/holes.
In all cases the ceramic has high resistance to ionic current at room temperature and the operating temperature is raised, usually to the range of 500 to 1100° C. The MIEC conductors may exhibit electronic (electron/hole) conduction at room temperatures.
Ceramic materials are known in the art as heating elements (e.g. SiC) and are also used as light sources in the form of Nernst glow lamps. In these cases an electrical current flows through the ceramic material. Nernst lamps were an early form of electrically powered incandescent lamps. Nernst lamps did not use a glowing tungsten filament. Instead, they used a ceramic rod that was heated to incandescence. The rod was an ionic conductor. In these cases a ceramic rod or strip, including a spiral strip was used. Unlike tungsten wire, the rod would not further oxidize when exposed to air. Therefore was no need to enclose it within a vacuum or noble gas environment. The burners in Nernst lamps could operate exposed to the air and were only enclosed in glass to isolate the hot incandescent emitter from its environment.
Developed by the German physicist and chemist Walther Nernst in 1897 at Goettingen University, these lamps were about twice as efficient as carbon filament lamps and they emitted a more “natural” light, more similar in spectrum to daylight. The lamps were quite successfully marketed for a time, although they eventually lost out to the more-efficient tungsten filament incandescent light bulb. One disadvantage of the Nernst design was that the ceramic rod was not electrically conductive at room temperature, so the lamps needed a separate heater filament to heat the ceramic hot enough to begin conducting electricity on its own. In the U.S., Nernst sold the patent to George Westinghouse who founded the Nernst Lamp Company at Pittsburgh in 1901.
Ceramic conductors such as SiC are used in furnaces for high temperature heating elements. They usually have a form of a spiral, and are used for operating the furnace in the temperature range 1000-1500° C. These exhibit electronic conductivity all the way down to room temperature. Purposeful ceramics tubes, such as for oxygen permeation, were not self-heated, but relied on external heating. The term self-heating means passing an electrical current through the ceramic tube which leads to heating of the tube.
A self-heated, dense ceramic tube for gas separation is disclosed. At least one dense ceramic tube is provided in an environment accessible to a mixture of gases, wherein the tube acting as a diffusion membrane for separating one gas from the mixture of gases. Two electrical contacts are provided which allow connecting a power supply to the ceramic tube and driving an electric current through the ceramic tube. The contacts can be in the form of short coils wrapped around the ceramic tube at difference places. They can have also other forms as mentioned below. When a voltage is applied to the contacts, an electrical current flows from one contact to the other through the ceramic tube. The current in the ceramic tube heats the tube. (The contacts need not contribute to heating). Thus, the tubes heat themselves. The hot tube allow separating one gas from the mixture of gases. The driving force for the diffusion is achieved by a pressure difference of the filtered substance generated by pumping at the side of the separated gas and/or pressing at the side of the mixture of gases.
Thus, it would be advantageous to enable significant electronic conductivity in dense ceramic tubes by self-heating to elevated temperatures to enable gas separation.